The EU and China were supposed to become the tag team of international climate diplomacy, buddying up after President Donald Trump announced his intention to pull the U.S. out of the Paris climate agreement. Brussels and Beijing, the world’s other big polluters, are publicly committed to tackling global warming.

Now this relationship is under strain. The main culprit? With the U.S. marginalized on this scene, the EU is left to carry the standard for rich countries — and China for the developing world.

The strain created by this dynamic has emerged behind closed doors at the COP23 climate summit here. The EU is pushing to level out the share of responsibility between rich and poor countries for man-made climate change and for reducing emissions. For its part, Beijing wants developing countries to have greater leeway to meet targets set out in the 2015 treaty.

The differences are likely to grow more stark at next year’s COP24, when the complex set of rules for meeting the goals of the Paris climate agreement is due to be finalized.

At the technical talks in Bonn that ended Tuesday, China pushed hard to demarcate responsibilities between developed and developing countries — called differentiation — throughout the text of the Paris climate agreement in a draft handed to ministers on Wednesday, according to negotiators and observers following the discussions. The political negotiations involving ministers run through Friday.

“China was putting differentiation in anywhere and weren’t agreeing to the text,” Mark Lutes, head of the World Wildlife Foundation’s delegation, said of last week’s technical talks.

China is particularly emboldened on the issue now that the U.S. — which has always been staunchly opposed to differentiation — has stepped to the side in the wake of Trump’s call to quit the Paris accord.

Bifurcation makes a comeback

The idea of treating rich and poor countries differently is called “bifurcation.” The Paris deal managed to paper over the issue by calling on everyone to pitch in and help slash greenhouse gases to the extent that they could.

Now developing countries are returning to the idea that wealthy countries should bear more of the emissions cuts. More than a century of their pollution, after all, is what many scientists say caused a lot of the problem of global warming. In response, developed nations point to burgeoning greenhouse gas emissions from countries like China, India and Brazil to say that everyone should shoulder the weight.

“We thought we put it away in Paris, that we went beyond that. But bifurcation is coming back,” said a European diplomat in Bonn, asking not to be named. “It’s quite a big fight and it’s a very important issue for the EU.”

The call to differentiate burdens and standards is especially loud in talks over the transparency and accounting system that will be used to keep track of emissions reductions, financial aid and other steps intended to tackle climate change. It is part of a broader set of rules that countries are negotiating at the COP23 to help them meet the Paris climate agreement’s voluntary goals.

‘We argue as good friends’

The EU and U.S. have long argued for a single, robust system to make sure everyone is measuring and reporting their emissions and aid in the same way. Without it, there’s no way to know, for instance, that one ton of CO2 cut in one country isn’t then emitted somewhere else. The poorest and most vulnerable countries are also keen to adopt a tough system, particularly to verify that developed countries fulfill their promise to spend $100 billion a year in public and private aid on climate change from 2020.